Conventional transmissive type liquid crystal displays (LCD) suffer low image contrast when the ambient environment is bright. That is, the color reproducibility is lower and the display is not sufficiently recognizable because the ambient light is brighter than the display light. Moreover, use of the backlight increases power consumption. Reflective liquid crystal displays comprise a reflector formed on one of a pair of substrates rather than a backlight so that ambient light is reflected from the surface of the reflector. The method is disadvantageous, however, in that the display is less visible when the ambient environment is dark.
In order to overcome the aforementioned problems, a liquid crystal display which realizes both a transmissive mode and a reflective mode in a transflective liquid crystal display device has been disclosed in, for example, U.S. Pat. Nos. 6,281,952 and 6,295,109, the entireties of which are hereby incorporated by reference. Each pixel of the transflective liquid crystal display splits into reflective (R) and transmissive (T) sub-pixels. Usually, the R and T area ratio is 4:1, in favor of reflective display. The transmissive display is used only in dark environments for power conservation.
FIG. 1A is a cross section of a conventional transflective liquid crystal display with a single LC cell gap. FIG. 1B is a cross section of a conventional transflective liquid crystal display with a double cell gap.
In FIG. 1A, a transflective LCD 10 with a single LC cell gap comprises an upper substrate 12 and a lower substrate 14 with a liquid crystal layer 16 interposed therebetween. The lower substrate 14 comprises a plurality of pixel electrode regions. Each pixel electrode region comprises a reflective electrode region 18 and a transmission region 20, separately defining a reflection region (R) and a transmission region (T). The upper substrate 12 comprises a transparent electrode 22 to serve as a common electrode. A first polarizer 26I is disposed on the upper substrate 12 opposing the liquid crystal layer 16. A first quarter wave plate 24I is disposed between the first polarizer 26I and the liquid crystal layer 16. A second polarizer 26II is disposed on the lower substrate 14 opposing the liquid crystal layer 16. A second quarter wave plate 24II is disposed between the second polarizer 26II and the liquid crystal layer 16. A backlight is provided under the lower substrate 14. The cell gaps at the R and T regions are the same, as shown in FIG. 1A. The cell gap is optimized for R region. In FIG. 1B, a transflective LCD 28 with a double LC cell gap is nearly identical to the transflective LCD 10 in FIG. 1A and for simplicity its detailed description is omitted. The transflective LCD 28 differs from the transflective LCD 10 in that the liquid crystal layer 16 of each pixel comprises two different thicknesses, i.e. the cell gaps are d1 and d2 for the R and T sub-pixels, respectively. In this approach, both the R and T sub-pixels have high efficiency. However, the response time of the T mode is four times slower than the response time of the R mode. Moreover, because this approach requires complicated structures and processes, the cell gap accuracy and uniformity can be poor. This can result in deteriorating LCD performance, such as variations in brightness and color.
It is desirable to overcome these and other problems of the prior art and to provide transflective LCDs including T and R regions with similar retardation changes that provide both regions with high light modulation efficiency.